Dental furnace

ABSTRACT

The invention relates to a dental furnace wherein a firing chamber is heated up in a first heating-up period at a first heating-up rate of more than 50° K/min, in particular more than 100° K/min, which heats the furnace to at least 1000° C., in particular to 1100-1250° C. The first heating-up period is followed by an intermediate heating period, which is at least five minutes long, in particular at least ten minutes long, the gradient or heating-up rate of which is adapted to the material to be sintered in the dental furnace ( 10 ), and wherein this is followed by an end heating-up period ( 44 ) during which heating up is effected at a heating-up rate of more than 30° K/min, in particular approximately 50° K/min, and wherein during this the furnace temperature is held for at least five minutes, in particular for at least 25 minutes, above the temperature toward the end of the first heating-up period, and wherein forced cooling of the furnace ( 10 ) is performed after this.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims foreign priority benefits under 35 U.S.C.§119(a)-(d) from German patent application ser. no. P 10 2008 012 578.4filed Mar. 5, 2008.

TECHNICAL FIELD

The invention relates to a dental furnace which is heated up in stagesand also cooled down to a method for heating up and cooling down, andmore particularly to such a furnace used for sintering dental materials.

BACKGROUND OF THE INVENTION

A dental furnace of this type and a method of this type have long beenknown. Precisely for sintering adapted dental materials it is importantto cause the heating up, the actual firing, and indeed the cooling downas well, to proceed according to a predetermined and reproducible schemein order on the one hand to ensure the required material compaction, buton the other hand also to ensure that the shrinkage takes placeuniformly to the entire extent.

For this purpose, the temperature in the interior of the dental furnaceis typically controlled with a precisely determined temperature profile.For this purpose, the heating elements are connected to a correspondingcontrol device, and a temperature sensor is normally used.

The temperature sensor is typically arranged in the upper region of thefiring chamber serving as working space, the restorations being placedthere.

Temperature sensors can therefore be arranged in or on the wall of thefiring space, and it is known to use special calibration devices toensure that the temperature in the interior of the dental furnacefollows a predetermined temperature profile.

On the other hand, the heat capacity of the introduced mass is aparameter that influences the heating-up profile of the dental material.The heating-up rate is typically lower if large masses are used, andhigher if small masses are used. In order to compensate for this effect,it is possible to detect the introduced mass beforehand and to providecalibration curves for different masses. However, this is complicatedand greatly dependent on the operator's care.

Moreover, the mass of the introduced dental material cannot usually beascertained exactly.

Therefore, it is known to work with a comparatively low heating-up ratein order to provide for the dental materials the possibility of bringingabout a homogeneous temperature compensation, irrespective of what massis present. Although this method is good in principle, it isdiametrically opposite to the desires in the dental laboratory to savecosts by means of a short production cycle.

Furthermore, U.S. Pat. No. 6,025,065 has, however, also disclosedcombining an extremely high heating-up rate of more than 1001C/min witha high temperature of 1300° C. to 1600° C. Although the sinteringfurnace therein is in principle extremely well suited to the rapidsintering of materials, the dimensional accuracy thereof is of lesserrelevance. Such a furnace is not suitable, however, for dentalmaterials.

OBJECTS AND SUMMARY OF THE INVENTION

Against this background, the invention is based on the object ofproviding a dental furnace which are particularly well suited to theproduction of dental materials.

The invention provides for heating up a dental furnace with a firingchamber at an extremely high first heating-up rate until a temperaturecorresponding to a presintering temperature has been reached. What canbe achieved by means of the presintering expedient according to theinvention is that the sintering material can be processed after thepresintering.

Surprisingly, by means of the rapid heating up according to theinvention at the first high heating-up rate, which is ended only at atemperature of 1000° C., 1100° C. or even 1200° C., the sintering cyclecan be significantly accelerated without disadvantages occurring in thecase of the final strength, on the one hand, or in the case of theaccuracy of fit, on the other hand.

Surprisingly, the observation even becomes apparent that the finalstrength is increased by this rapid heating up in comparison with aslower heating up.

According to the invention, the first heating up with the high firstheating-up period is followed by an intermediate heating period, theheating-up rate or temperature gradient of which is significantlysmaller than that of the initial heating-up period. By way of example,the temperature gradient during the intermediate heating period can be2° K, 3° K, 5° K or 10° K/min.

After the intermediate heating period, the duration of which can beadapted to the requirements in wide ranges and can be for example 5 min,10 min, 20 min or 30 min, an end heating-up period is provided, thetemperature gradient or heating-up rate of which is likewisesignificantly higher and can be for example at least 20° K/min, butpreferably approximately 50° K/min.

The temperature difference between the temperature toward the end of theintermediate heating period and the end temperature toward the end ofthe end heating-up period is comparatively small and is for examplesomewhat more than 100° C., or 200° C. for example, without anydeterioration in the accuracy of fit.

By contrast, the accuracy of fit is significantly improved by the lowheating-up rate during the intermediate heating period.

According to the invention it is particularly expedient for the end ofthe heating-up period to be followed by a holding period, during whichthe temperature in the firing chamber is held substantially at the endtemperature toward the end of the heating-up period or just below that.The final density and the strength of the dental material can besignificantly improved by this measure.

The realization of a particular dental furnace is particularly expedientfor the configuration of the temperature profile according to theinvention. In order to be able to realize the high temperature gradientdesired, a dental furnace according to the invention preferably has alow heat capacity between the heating elements and the firing space,which heat capacity may be composed for example of a rather thininsulation material, for example quartz glass. The comparativelyeffective heating elements would permit the dental furnace to be heatedup to 1600° C. from room temperature within somewhat more than 10 min,where it is understood that a particular temperature profile is soughtaccording to the invention.

The dental furnace according to the invention takes account particularlyof the fact that the sintering in the case of the dry sintering takesplace in three stages, namely an initial stage, an intermediate stageand an end stage, wherein the sintering rate, that is to say thecontraction of the material per unit time, is the highest in theintermediate stage, such that for example 90% of the final density canbe achieved at the end of the intermediate stage.

According to the invention it is preferred to allow the initial stage tobe undergone as early as during the presintering, such that thesintering rate during sintering to completion only undergoes theintermediate and end stages.

In this connection it is expedient according to the invention if a lowheat capacity is also used for the thermal insulation, wherein it isalso possible, for example, to keep the thermal insulation layer at adistance from the heating element, such that the capacity no longerplays a part. This additionally has the particular advantage that thecooling down of the heating elements can be significantly improved byconvection. By way of example, it is also possible to realizerear-ventilated heating elements, that is to say heating elements inwhich the air flow with the furnace hood open also contributes to thecooling of the heating elements from the side remote from the firingchamber.

The dental furnace according to the invention can be a furnace with afiring chamber which can be removed from a substructure. In this case,either a lifting mount or a pivoting mount or a combination of thesemounts is possible. One example of such a mount can be seen from U.S.Pat. No. 5,788,485.

In addition to the convection cooling it is also possible to use activecooling by means of a fan in order in this respect in any case toachieve the desired cooling-down rate. This holds true particularly whena dental furnace in a traditional form with a firing chamber door isused, a dental firing furnace with a flat bearing surface and aremovable firing hood being preferred.

According to the invention, the temperature and duration of theprecompaction, by means of the presintering, can be adapted to therequirements within wide ranges. The presintering is preferably effectedin a separate process step beforehand on the dental block. By way ofexample, the presintering can be effected at the final temperature ofthe first heating-up period, that is to say for example at 1100° C. orat 1250° C. This presintering has the advantage that the presinteredmaterial can still be mechanically processed since the hardness issignificantly lower than in the material sintered to completion.

By comparison with high-temperature sintering furnaces known per se, itis particularly preferred according to the invention that the cycle timeis significantly reduced, by comparison with the typical 8 to 10 hoursfor sintering to completion and cooling down in the case of knownhigh-temperature sintering furnaces.

According to the invention, it is possible, by contrast, to reduce thetotal cycle time to less than three hours, including the cooling down,and in a modified embodiment of the solution according to the invention,the total cycle time can be reduced to less than 90 min despite the useof high-strength dental ceramic with firing temperatures of more than1500° C.

An alternative embodiment provides for using, instead of a sinteringfurnace, a microwave furnace for the realization of the dental furnaceaccording to the invention.

The firing curve accelerated according to the invention is distinguishedin diagrammatic representation by a “block form with shoulders”. Thefirst heating-up period is extremely short with a large temperaturegradient, as is the end cooling-down phase with a likewise steeptemperature gradient. A temperature phase above 1100° C. that issignificantly lengthened in comparison with the total length of theheating curve is thus available, which can then be optimized accordingto the invention. Thus, it is expedient according to the invention ifthe “high-temperature phase” takes up 68%, that is to say in thisrespect just below 70%, of the total firing cycle in the case of a shortfiring curve of less than three hours, and even above 80% in the case ofa standard firing curve, in each case relative to the heating up fromroom temperature and the cooling down to room temperature and theduration for which the firing temperature of more than 1100° C. iscomplied with.

By obviating kiln furniture, it is possible to further reduce the heatcapacity or thermal mass in relation to kilns having kiln furniture. Itis also particularly expedient according to the invention if, instead ofpassive cooling, active cooling is also effected precisely in theinterspace between the thermal insulation and the heating element, andalso within the open firing space, such that the desired cooling-downrate can be achieved.

It is preferred in this connection if in this respect rear-ventilatedfiring chamber is embodied expediently in terms of flow, such that theactive ventilation can be realized with a comparatively low fanrotational speed and thus very quietly.

For this purpose, two flow ducts are preferably provided, namely arear-ventilation flow duct and a flow duct through the firing chamber,wherein it is understood that the convection cooling can also beproduced at least in part—precisely at high temperatures.

In a further preferred configuration it is provided that the sinteringmaterial to be sintered is heated up in a first heating-up period at aheating-up rate of more than 50° K/min, wherein, between the end of thefirst heating-up period and the beginning of an end heating period thereis an intermediate heating period at a heating-up rate significantlylower than 50° K/min, in particular less than 10° K/min, and wherein thefirst heating-up period and the end heating period are set in amaterial-independent manner, and wherein the intermediate heating periodis defined, with regard to its length and its heating-up rate, in amanner dependent on the material to be sintered.

In a further preferred configuration it is provided that the maximumtemperature in the heating chamber is approximately 1600° C. and thefurnace can be heated up to 1600° C. proceeding from room temperature inits heating chamber, within less than 30 minutes.

In a further preferred configuration it is provided that theintermediate heating-up rate is lower than the initial heating-up rateby approximately a power of 10, in particular by a factor of 10 to 50.

In a further preferred configuration it is provided that a heatingchamber of the furnace is surrounded by a heat-resistant insulation, inparticular a pressed shaped part composed of fiber, the wall thicknessof which is preferably between 15 and 25 mm.

In a further preferred configuration it is provided that theintermediate heating period is chosen in terms of the temperature and/orthe time such that it covers the intermediate stage of the sinteringprocess of the dental material to be sintered, in which the sinteringrate, plotted against the temperature/time, is the highest.

In a further preferred configuration it is provided that the dentalmaterial to be fired is presintered and/or precompacted and prior to theactual sintering has a strength which is significantly lower than, inparticular less than half the magnitude of, the final strength of thesintering material.

In a further preferred configuration it is provided that the initialheating-up rate is chosen such that it corresponds to the maximumheating-up rate at which no overshoot arises upon the transition fromthe initial heating-up period to the intermediate period, but is atleast 50° K min⁻¹.

In a further preferred configuration it is provided that, after aholding time has elapsed, the furnace cools down at a first cooling-downrate, which is less than the heating-up rate of the initial heating-upperiod and greater than the heating-up rate of the intermediateheating-up period, and wherein a higher, second cooling-down rate is setafter this.

In a further preferred configuration it is provided that the dentalfurnace is a microwave furnace.

In a further preferred configuration it is provided that a methodwherein a heating chamber is heated up in a first heating-up period at afirst heating-up rate of more than 50° K/min, in particularapproximately 100° K/min, which heats up the furnace to at least 1000°C., in particular to 1100° C. to 1250° C., wherein the first heating-upperiod is followed by an intermediate heating period, which is at leastfive minutes long, in particular at least ten minutes long where thegradient or heating-up rate thereof is subsequently adapted to thematerial to be sintered in the furnace, and wherein this is followed byan end heating-up period, during which heating up is effected at aheating-up rate of more than 20° K/min, in particular approximately 50°K/min, and wherein during this the furnace temperature is held for atleast five minutes, in particular for 25 minutes, above the temperaturetoward the end of the first heating-up period, and wherein forcedcooling of the furnace is performed after this.

In a further preferred configuration it is provided that a methodwherein the sintering material to be sintered is heated up in a firstheating-up period at a heating-up rate of more than 50° K/min, wherein,between the end of the first heating-up period and the beginning of anend heating period, preheating is effected for an intermediate heatingperiod at a heating-up rate significantly lower than 50° K/min, inparticular less than 10° K/min, and wherein the first heating-up periodand the end heating period have been or are set in an object-independentmanner, and wherein the intermediate heating period is defined, withregard to its length and its heating-up rate, in a manner dependent onthe material to be sintered.

In a further preferred configuration it is provided that dental toothreplacement material is sintered in dry fashion or in liquid sintering,wherein the sintering material comprises an oxide ceramic which iscomposed, in particular, of ZrO₂, of Al₂O₃ and compositions thereof andcomprises, in particular, a doping auxiliary.

In a further preferred configuration it is provided that, starting whenthe holding time has elapsed, forced cooling of the furnace isperformed, which leads to the cooling down of the tooth replacementmaterial to a removal temperature, in particular of approximately 400°C., in less than 60 minutes, in particular 20 to 60 minutes.

Further advantages, details and features will become apparent from thedescription below of an exemplary embodiment with reference to thedrawing, in which:

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a schematic and partially cut-away view of a dental furnaceaccording to the invention; and

FIG. 2 shows two exemplary embodiments of firing curves for the dentalfurnace according to the invention.

DETAILED DESCRIPTION

The dental furnace 10 illustrated in FIG. 1 has a furnace hood 12, whichis mounted on a furnace lower part 16 by means of a pivoting articulatedjoint 14. The lower part 16 has on its top side a bearing surface 18,which is intended for receiving the dental material to be fired. Afiring chamber 20 is provided in the furnace hood 12, and it extends inthe manner of a rather flat cylinder and, with the furnace hood 12closed, is closed off at the bottom by the bearing surface 18, such thatthe bearing surface 18 forms the bottom of the firing chamber 20.

The firing chamber 20 is surrounded annularly or spirally by heatingelements 22.

According to the invention, particularly powerful heating elements areprovided, which are designed such that they are fundamentally able toheat up the furnace from room temperature to 1600° C. withinapproximately a quarter of an hour. The dental furnace accordingly has amax. temperature gradient of 120° K/min.

The heat capacity of the firing chamber 20 and of the parts surroundingthe firing chamber 20 is low.

The heating elements 22 are additionally significantly rear-ventilated.An air space 28 is provided for this purpose, said air space surroundingthe heating elements 22 and thus the firing space 20 on all sides. Theair space 28 is extremely large and takes up a considerable part of theinterior of the furnace hood 12. The furnace hood 12 has a thermalinsulation layer 30 surrounding the air space 28, which layer—eventhough this cannot be seen in FIG. 1—can also have perforations formingair ducts in order to facilitate the air flow via air outlets 32 in theupper region of the furnace hood 12.

The dimensioning both of the air space 12 and of the thermal insulationlayer 30, can be adapted to the requirements within wide ranges, it alsobeing possible to work with an extremely thin thermal insulation layerof just 15 mm, for example.

The dental material preferably provided is applied to the bearingsurface 18 according to the invention. After the furnace hood 12 hasbeen closed, the heating element 22 is switched on with max. power, suchthat the firing chamber 20 is heated extremely rapidly to 1200° C., forexample. This temperature may substantially correspond to thepresintering temperature. After this, during an intermediate heatingperiod, the temperature is increased with a small temperature gradientuntil a temperature of approximately 100° C. below the final temperaturehas been reached. After this, the temperature is increased extremelyrapidly again to the final temperature and after this is held for apredetermined time duration, wherein the holding time may depend both onthe applied amount of dental material and on further parameters.

After this, the temperature is reduced, to be precise preferably firstlywithout active cooling, wherein the active cooling is switched on whenthe presintering temperature has been reached again, such that thecooling proceeds more rapidly starting from said temperature until roomtemperature is reached.

As an alternative, in an even more highly accelerated firing cycle, thecooling down can take place directly after the holding time with activecooling, such that the cooling-down period overall is shortened further.

An initial heating-up period 40, which is ended at approximately 1100°C. in accordance with curve 2, is followed by an intermediate heatingperiod 42, which performs heating up to approximately 1350° C.

After this, an end heating-up period 44 is provided, which increases thetemperature to 1500° C., which final temperature is reached 100 minafter the beginning of the firing cycle in the case of “curve 2”.

During the holding time 46 of approximately 30 min, the temperature isheld at 1500° C. and, during the initial cooling-down period 48, thetemperature is lowered to 1100° C. within less than 30 min.

After this, the end cooling period 50 is provided, by means of which thetemperature is lowered to room temperature within likewise somewhat lessthan half an hour.

This firing cycle according to the invention is illustrated in twoembodiments in FIG. 2. Accordingly, the following firing curve resultsfor the curve designated as “curve 2”:

Temperature/ Rate/ Time/ Total/ Ramp C. K min⁻¹ min min 0 25 1 1100 10010.75 10.75 2 1350 3 83.33 94.08 3 1500 50 3.00 97.08 4 1500 0 30.00127.08 5 1100 −15 26.67 153.75 6 200 −50 18.00 171.75Firing Curve 2

Temperature/ Rate/ Time/ Total/ Ramp C. K min⁻¹ min min 0 25 1 1250 10012.25 12.25 2 1350 5 20.00 32.25 3 1500 50 3.00 35.25 4 1500 0 25.0060.25 5 1100 −25 16.00 76.25 6 200 −46.6 19.31 95.56Firing Curve 1

The total firing curve is reduced even further in the modifiedembodiment in accordance with “curve 1” to approximately 95 min, whereina substantially trapezoidal curve profile is provided in both cases,each having a high initial heating-up rate and an equally or almostequally high end cooling-down rate. This is also evident from the tableabove.

In contrast to the firing cycle in accordance with curve 2, an overshootof the heating power to a temperature of, for example, 50° C. above thetemperature of the holding time 46 is provided in the case of curve 1.

Surprisingly, the strength is increased by the rapid heating-up to thepresintering temperature, or alternatively to 1250° C., while there isno measurable influence on the accuracy of fit. By contrast, theaccuracy of fit and hence the distortion are improved by the slowheating up during the intermediate heating period, while the strength isnot adversely affected. By contrast, the end heating up, for example by150° C., to the end heating up temperature, which may lie between 1500°C. and 1600° C., has no particular influence on the strength and noinfluence at all on the accuracy of fit.

By contrast, the comparatively long holding time has a very greatinfluence on the strength and in particular also on the final density,and the relatively slow cooling-down to the presintering temperature asprovided in accordance with curve 2 also has a measurable influence onthe strength, while the subsequent cooling down to room temperature haspractically no further influence on the strength or the density.

According to the invention, it is therefore possible to realize a dentalfurnace with a short firing cycle, yet particularly attractive firingresults.

While a preferred form of this invention has been described above andshown in the accompanying drawings, it should be understood thatapplicant does not intend to be limited to the particular detailsdescribed above and illustrated in the accompanying drawings, butintends to be limited only to the scope of the invention as defined bythe following claims. In this regard, the terms as used in the claimsare intended to include not only the designs illustrated in the drawingsof this application and the equivalent designs discussed in the text,but are also intended to cover other equivalents now known to thoseskilled in the art, or those equivalents which may become known to thoseskilled in the art in the future.

What is claimed is:
 1. A total firing method for sintering a dentalmaterial comprising: heating a heating chamber in a first heating-upperiod at a first heating-up rate of more than 50° K/min, which heats upthe furnace to at least 1000° C., continuing to heat the chamber in anintermediated heating period, wherein the first heating-up period isfollowed by the intermediate heating period, which is at least fiveminutes long, and continuing to heat the chamber in an end heating-upperiod, wherein the intermediate heating period is followed by the endheating-up period, during which heating-up is effected at a heating-uprate of more than 20° K/min, wherein the end heating-up period has arate that is slower than the first heating-up rate, and holding thetemperature in the end heating period for at least five minutes, above atemperature that has been measured at the end of the first heating-upperiod, and force cooling the furnace after the end heating period,wherein the total firing method takes place over a time span from astart of the first heating-up period at room temperature to the coolingdown of the heating chamber to room temperature, wherein the heatingchamber has a temperature higher than 1100° C. at least 68% of the timespan of the total firing method, wherein during the intermediate heatingperiod about 90 percent of final density of the dental material beingsintered is achieved.
 2. The method as claimed in claim 1, wherein,starting when the holding time has elapsed, forced cooling of thefurnace is performed, which leads to the cooling down of the dentalmaterial to a removal temperature of approximately 400° C., in less than60 minutes.
 3. The total firing method as claimed in claim 1, whereinthe first heating-up rate is approximately 100° K/min and wherein thefurnace is heated up to 1100° C. to 1250° C. in the first heating-upperiod, wherein the intermediate heating period is at least ten minuteslong, wherein the end heating-up period is effected at a heating-up rateof approximately 50° K/min, and wherein during the end heating-up periodthe furnace temperature is held for 25 minutes above the temperaturetoward the end of the first heating-up period.
 4. The method accordingto claim 2, wherein the cooling of the furnace (10) is performed in 20to 60 minutes.
 5. A method wherein a dental material to be sintered isheated up in a first heating-up period at a heating-up rate of more than50° K/min, continuing to heat the dental material in an intermediateheating period at a heating-up rate lower than 50° K/min, wherein,between the end of the first heating-up period and the beginning of anend heating period, preheating is effected in the intermediate heatingperiod wherein the first heating-up period and the end heating periodare set in an object-independent manner, wherein the intermediateheating period is defined, with regard to its length and its heating-uprate, in a manner dependent on the material to be sintered, whereinduring the intermediate heating period about 90 percent of final densityof the dental material being sintered is achieved, and wherein the endheating-up period has a rate that is slower than the heating-up rate inthe first heating-up period.
 6. The method as claimed in claim 5,wherein the dental material comprises a dental tooth replacementmaterial and is sintered in dry fashion or in liquid sintering, whereinthe dental material comprises an oxide ceramic which is composed ofZrO₂, of Al₂O₃ and compositions thereof and comprises a dopingauxiliary.
 7. The method according to claim 5, wherein the heating-uprate of the intermediate heating period is less than 10° K/min.